Overview

Bantry Bay is the largest of the long marine inlets in south-west Ireland. It is approximately 35 km long, running in a south-west to north-easterly direction. The entrance to the bay is approximately 10 km wide, narrowing to 3-4 km at its head. Bere Island, situated on the north shore adjacent to Castletownbere, and Whiddy Island lying near the head of the bay on the southern shore are the two largest islands in the bay. Rivers enter the bay at Melagh River near Bantry, Owvane River at Ballylicky, Coomhola River at Dromkeal, Glengarriff River at Glengarriff, Adrigole River at Adrigole.

Dunmanus Bay to the south of Bantry Bay is divided by the Sheep’s Head and Mizen peninsulas. The bay is nearly 7 km wide from Sheep’s Head to Three Castle Head and 25 km long from mouth to its head at Four Mile Water. The largest islands within the bay are Carbery, Furze, Horse and Cold Islands. The Durrus River drains into the bay at Durrus.

Location of Bantry and Dunmanus Bays, Cork in the southwest of Ireland.

INFOMAR Survey History

INFOMAR surveys of Bantry and Dunmanus Bays began in 2004 when the Celtic Explorer mapped a small area of seabed around Dursey Head as part of a larger survey of the southwest priority area. In 2006, the Celtic Explorer was used to map the outer reaches of both bays. Later in the year, IMAR survey were contracted to continue the survey, covering the middle and inner bay up to Whiddy Island and the channel between the Beara peninsula and Bear Island in Bantry Bay.

Also in 2006, a Tenix LADS lidar survey mapped the nearshore and shallow waters in both bays. This was particularly effective at Whiddy Island, the channel between the Beara peninsula and Bear Island and at Glengarriff. September 2007 saw the remaining seabed of both bays surveyed using the Celtic Voyager to achieve complete coverage of both bays.

Coverage from all surveys involved in mapping Bantry and Dunmanus Bays for the INFOMAR project

Details of surveys undertaken in Bantry and Dunmanus Bays (Click image for more detail).

Bathymetry (Water Depth)

Bathymetry of Bantry and Dunmanus Bay (Click image for more detailed map).

In general terms, the bathymetry of the area is as expected with relatively shallow areas within the bays which deepen moving offshore. Water depths range from 0 metres to 76 metres to the southwest of the bays. It is interesting to note that due to the structural control on the bays, the rocks are often steeply dipping into the ocean resulting in sea cliffs. Where this occurs, particularly on the outer Sheep’s Head, the water depth increases rapidly to depths of 40 to 50 metres.

In contrast, the channel to the north of Bere Island is relatively shallow with water depths closer to 20 metres. However, this depth is sufficient for the large numbers of fishing and recreational vessels that use Castletown Bere harbour to shelter from rough sea conditions from the southwest and south.

For a more detailed chart of the bathymetry of Bantry and Dunmanus Bays click here

Lidar

In 2006, a Tenix LADS lidar survey mapped the nearshore and shallow waters in both bays. This was particularly effective at Whiddy Island, the channel between the Beara peninsula and Bear Island and at Glengarriff.

The oil terminal on Whiddy Island. Lidar shaded relief on left and Lidar aerial photos on right.

Dunmanus Bay Coastal areas less than 20 metres deep extending from Foilavaun Point on the north west coast to Dunbeacon Harbour at the head of the bay and west along the south coast to Dooneen Point. Depths to a maximum of 20 metres were achieved, however in places the depths achieved were considerably less due to the high levels of turbidity.

Bantry Bay Coastal areas less than 20 metres deep extending from Bear Island on the north west coast to Bantry at the head of the bay and west along the south coast to Trasloosh Landing, including Glengarriff Harbour and Adrigole Harbour.

All of our Charts for Bantry & Dunmanus can be downloaded in pdf format from this page.

Shaded Relief

The 3D appearance is achieved using software called Fledermaus. By using some vertical exaggeration, artificial sun-shading (usually as if there is a light source in the nw 315°) and colouring the depths using various colour maps, it is possible to highlight the subtle relief of the seabed. This helps us to quickly understand the variation in depths.

A shaded relief image of the pipeline and pilings for the jetty at the former Conoco Phillips oil storage facility at Whiddy Island where the Betelgeuse oil tanker exploded while docked at the deep water jetty used for transferring oil to the terminal on shore in 1979 (Click image for more detailed map).

All of our Charts for Bantry & Dunmanus can be downloaded in pdf format from this page.

Backscatter

Multibeam Systems also collect additional information, including the strength of the acoustic signal (or return) from the seafloor. This is known as Backscatter. Differing seafloor types, such as mud, sand, gravel and rock will have different Backscatter values depending on the amount of energy they return to the sonar head. Rocky areas will typically have high returns while soft sediments like mud are more likely to absorb energy and have low Backscatter returns. These differing values are used to generate a grey-order image (i.e. dark for high returns, bright for low returns) of the seabed which can be used to examine the nature of the seafloor.

MBES backscatter image of the outer reaches of Bantry Bay and approaches to Castletownbere. Rock outcrops appear dark with surrounding sediments various shades of grey depending on grain size and texture. The NNE to WSW ‘grain’ of the regional geology is especially apparent (Click image for more detailed map).

Ground Truthing/Seabed Sampling

To verify the results of remotely sensed sonar data, it is important to collect physical sediment samples from the seabed. These may be from the surface where grabs are used or may penetrate through the seabed and retain the vertical structure of the sediment by using various coring methods. These samples are also critical for the confirming seabed classifications.

A range of seabed sampling has been undertaken in Bantry and Dunmanus Bays, both historically and under the INFOMAR project. These include grab samples, vibrocores and video footage. The locations of the grab samples have been mainly determined by the seabed classification made from the multibeam data.

Seabed sampling locations from Bantry and Dunmanus Bays where Van Veen grab, Day grab and Box core instruments were used to ground truth INFOMAR datasets (Click image for more detailed map).

Seabed Classification

Seabed classification is an advanced process in which a backscatter image is interrogated using special software in order to divide it into areas which have similar characteristics. Areas which have a similar nature are grouped together into classes. These classes are colour coded and developed into seabed classification charts. The classes on the chart are then physically sampled in order to ground truth the classification. This allows INFOMAR to match different sediment types with their corresponding colour class thereby producing classified geological maps of the seafloor.

The classification of the multibeam dataset from Bantry and Dunmanus Bays resulted in the creation of a 5 class classification divided into two types of rock, reflecting the different textures observed from rock outcrops in the bay. Three more classes divided the sediments into Gravels and Coarse Sand, Coarse to Medium Sand and Fine Sand to Mud.

Seabed Habitats \ Seabed Substrate

The seabed substrate information has been derived from a combination of the analysis of geophysical data (multibeam echosounder bathymetry and backscatter) and groundtruthing data acquired as part of the INSS and INFOMAR seabed mapping programmes.

Geology

Bantry Bay Geology
The geology of Bantry Bay comprises mostly sandstone, with some occurrences of mudstone. The biggest rock formation in the area, the famous Old Red Sandstone, was formed hundreds of millions of years ago during the Devonian period. The structure of the rock formations here comprises a series of folds, like a folded tablecloth, with their axes running broadly northeast to southwest. As a result the general trend of the peninsulas and bays is similar. Many of the underwater rock outcrops, as seen here in the seabed data, take the appearance of ridges with the same orientation, showing that this is a major structural trend in the landscape – both on land and in the seabed.

Dunmanus Bay Geology

The geology of the Dunmanus Bay area is dominated by the Old Red Sandstone, a famous rock formation that was formed during the Devonian Period. The rocks in this area are folded, like a tablecloth that has been pushed together at both ends, with the axes of the folds running broadly northeast to southwest – these structures may be responsible for the shapes of the peninsulas and their intervening bays. This trend in direction is also apparent in the rock outcrops in the seabed data, as they occur as ridges running in a northeast-southwest direction.